Formation of tungsten monocarbide from a molten tungstate-halide phase by gas sparging

ABSTRACT

Tungsten monocarbide is prepared by sparging a molten composition comprising an alkali metal halide and an oxygen compound of tungsten with a gas comprising a gaseous hydrocarbon, particularly methane.

This application is a continuation-in-part of pending application Ser.No. 544,299, filed Oct. 21, 1983 .Iadd.now abandoned.Iaddend..

This invention relates to a process for preparation of tungstenmonocarbide (WC) from oxygen compounds of tungsten.

Recent technological advances have resulted in an ever increasing demandfor materials that are capable of delivering constant chemical andphysical performances under a variety of extreme operating conditions.For example, the development of supersonic aircraft and rockets is basedupon the availability of such materials.

One material of relatively recent importance is tungsten. Tungstenpossesses a high melting point and is useful in its elemental form,particularly as an alloy for high-temperature applications. Tungsten isalso useful in the form of tungsten monocarbide for use in abrasives andcutting instruments. As the demand for increased availability of suchmaterials has advanced, so has the necessity for improved techniques fortheir production and recovery.

In the past, recovery of tungsten from its ores was an expensive andrelatively complex process involving both physical and chemicalseparation procedures. Tungsten was usually recovered either fromwolframite, (Fe,Mn)WO₄, or scheelite, CaWO₄, by variouspyrometallurgical and extraction procedures. The carbide final productwould then be obtained by reacting the purified metal with carbon inanother sequence of reactions.

The predominant commercial process for the preparation of tungstencarbide from wolframite or scheelite concentrates requires many stepsbecause the preparation of high purity tungsten powder is necessarybefore synthesis of tungsten carbide.

In the commercial production of tungsten monocarbide by directsynthesis, tungsten powder is mixed with 6.2-6.9% carbon. This amount ofcarbon is 2-10% in excess of the stoichiometric amount in WC (6.13%) andis required to decrease the oxygen content. The mixture is usuallyblended in a rotary blender containing steel or tungsten carbide balls.Typically one-half of the tungsten powder will be added to all of thecarbon, blended for several hours, and this is followed by the additionof the remaining tungsten powder and additional several hours ofblending. Careful blending procedures are required, particularly in thecase of coarse tungsten powder, because of great disparity in density ofthe constituents and the size effects of the particles. Thetungsten-carbon mixture is next put into graphite boats and passedcontinuously through a tube furnace. The furnace may be either gas firedor electrically heated, but must be maintained in the temperature rangeof 1,400° to 1,600° C. Hydrogen is usually introduced into the dischargeend of the tube and is burned off at the charging end. The time perfurnace cycle is 4 to 5 hours.

The major disadvantage of this process is the requirement for highpurity tungsten powder as the feed material for synthesis of thecarbide. The complete process for preparing monocarbide from oreconcentrates includes the following steps: (1) pressure digestion todecompose the ore, (2) solution purification, (3) solvent extraction,(4) ammonium hydroxide stripping, (5) ammonium paratungstatecrystallization, (6) formation of tungsten metal by hydrogen reduction,and (7) carbide synthesis.

Aluminothermic reduction is also used to produce WC from oreconcentrates. This process consists of blending the concentrates,aluminum, calcium carbide and other reagents in a tear-down carbon linedrefractory furnace. A thermite reaction is started and the whole massbecomes molten, thereby producing a slag and an iron-manganese alloycontaining WC crystals. The furnace is cooled in about 10-14 days andthe slag and menstruum separated. The menstruum is then crushed andleached to yield a coarse WC product. The coarse WC crystals next areground in a WC rod or ball mill and yield a commercial WC product.

This technique, however, is also disadvantageous in that it requires alarge amount of energy for size reduction and leaching the menstruum,and also for the size reduction of the recovered WC crystals.Additionally, the carbide thus formed is not suitable for preparing manycemented carbide products.

U.S. Pat. No. 3,482,941 to Palm discloses another method for recoveringtungsten in the form of tungsten carbide from scheelite by heating orewith silica and carbon. This process is said to reduce the amount ofditungsten carbide which is commonly formed when tungsten carbide isprepared at elevated temperatures.

U.S. Pat. No. 4,256,496 to Brandstatter discloses an additional processfor the recovery of metal ores utilizing a solid state reduction andcarburization reaction. The disclosed process utilizes physicalseparation techniques, such as gravity separation employing waterelutriation. The separation is based upon the different physicalproperties of metal carbides and various gangue oxide constituents ofores at elevated temperatures.

It has now been found, according to the present invention, thatpreparation of tungsten monocarbide from oxygen compounds of the metalmay be efficiently and economically accomplished by subjecting a melt ofan alkali metal halide and the tungsten compound to sparging with ahydrocarbon gas. The process of the invention has been found to beparticularly applicable to preparation of tungsten monocarbide fromtungstic oxide or tungstates such as sodium tungstate and ammoniumparatungstate.

The preferred alkali metal halide will generally be sodium chloridesince it is usually most readily available and economical. However, thealkali metal halide may also consist of fluorides or bromides of sodium,potassium, or lithium. In addition, mixed metal compounds such as NaAlFor KAlF may also be used.

Proportions of the alkali metal halide and the tungsten compound are notcritical, provided the amount of halide is sufficient for readyformation of a homogenous melt for treatment with the hydrocarbon gas.The percent of tungsten (in percent WO₃ by weight) will generally rangefrom about 5 to 30. Formation of the melt is accomplished by simplyheating an admixture of halide and tungsten compound to a temperatureand for a time sufficient to form a melt of sufficient fluidity forsparging with the hydrocarbon gas. A temperature of about 900° to 1100°C. and a time of about 3 to 8 hours is usually sufficient.

Formation of the melt, as well as the sparging, may be carried out in acrucible of any conventional refractory material such as graphite, orceramics such as alumina, magnesia, zircon or silicon carbide. However,it has been found that refractory metal alloys, such as Inconel, areparticularly effective in the process of the invention.

The preferred hydrocarbon gas will generally be methane, or natural gas.However, other hydrocarbons, such as ethane, acetylene, propane orbutane may also be used. In addition, mixtures of the hydrocarbon gas,or gases, with H₂ or CO may be used. Furthermore, other reductants suchas powdered charcoal, coke breeze or calcium carbide may be added to thesodium chloride melt to facilitate reduction during the spargingprocess. It has also been found that addition of small amounts of alkaliborates, carbonates, fluorides or hydroxides to the melt may assist inreduction and crystal growth.

Sparging is by conventional means using sparging tubes of any suitablyinert material such as graphite, alumina or Inconel. A temperature ofabout 900° to 1100° C. and a methane flow rate of about 4 to 12 litersper minute per liter of melt for a period of about 3 to 8 hours isusually sufficient to give substantially complete conversion of tungstento WC. A vacuum or inert atmosphere may also be beneficial in thesparging process.

The WC product has a density greater than the melt, and forms a separatephase. The phases are readily separated by decanting thetungsten-depleted phase from the melt, after which the WC-richcomposition is cooled and water leached to remove adhering salt from theWC product. The WC product may then be further purified by leaching withdilute HCl solution, followed by dilute caustic solution, and waterwashing, and is then dried. The dried WC product may be further purifiedby treatment for removal of any free carbon by a conventional heavyliquid treatment, resulting in gravity separation of the carbon from theheavier WC. Diiodomethane has been found very effective for thispurpose, although other heavy liquids such as acetylene tetrabromide maybe used. The free carbon can also be removed by floation or gravitymethods.

The process of the invention has been found to be particularly effectivein preparation of WC from concentrates of tungsten ores such asscheelite or wolframite. A sodium tungsten-sodium chloride compostion isinitially prepared by means of the process of U.S. Pat. No. 3,373,097,the disclosure of which is incorporated herein by reference. In thisprocess a melt comprising the ore concentrate, sodium chloride andsodium silicate is held at elevated temperature for a time sufficient toform separate halide and silicate phases. Sodium tungstate is formed bythe reaction (in the case of wolframite)

    2(Fe,Mn)WO.sub.4 +3Na.sub.2 SiO.sub.3 →2Na.sub.2 WO.sub.4 +Na.sub.2 (Fe,Mn).sub.2 Si.sub.3 O.sub.9

and concentrates in the halide phase, with gangue materialsconcentrating in the silicate phase. The phases are separated, and theresulting NaCl-Na₂ WO₄ composition is then treated according to theprocess of the present invention for formation of WC.

The invention will be more specifically illustrated by the followingexamples.

EXAMPLE 1

Feed material-Colorado wolframite concentrate

    ______________________________________                                        Analysis, pct                                                                 WO.sub.3 Fe.sub.2 O.sub.3 MnO                                                 69.1 9.1 13.5                                                                 High temperature phase separation                                                    Charge  grams                                                          ______________________________________                                               Wolframite                                                                            484                                                                   NaCl    704                                                                   Na.sub.2 SiC.sub.3                                                                    291                                                                           1,479                                                          ______________________________________                                    

The charge was melted for 2 hours at 1,075° C. in a silicon carbidecrucible. Molten charge was removed from the furnace and the two moltenphases were decanted into separate slag pots. The upper halide phase wasfluid and easily separated from the lower silicate phase which was veryviscous.

    ______________________________________                                        Analysis of the phases, pct                                                            Wt. g WO.sub.3   FeO    MnO                                          ______________________________________                                        Halide     980     31.4       0.05 0.17                                       Silicate   440     4.5        9.3  13.4                                                  1,420                                                              ______________________________________                                    

The halide phase containing 92 pct of the WO₃ in the wolframite wastreated by methane gas sparging.

Gas Sparging of the Halide Phase

A 600-gram charge of the halide phase was melted in a 3" I.D.×7" highgraphite crucible at 1,100° C. A graphite tube with four 1/16" holes atthe bottom was used as a gas sparger. Methane was passed through thetube for 6 hours at a rate of 4 liters/min STP. The gas was turned off,the sparging tube removed and the remaining molten salt in the cruciblewas decanted into a slag pot. The cooled crucible was then water leachedand scraped to recover the WC product. The WC product was leached indilute HCl and NaOH solutions, and free carbon removed by heavy liquidtreatment.

    ______________________________________                                        Results                                                                       ______________________________________                                                WC recovered = 128 g                                                          W recovered as WC, pct = 81                                                   Final melt, g = 310                                                           Final melt, pct WO.sub.3 = 10                                                 Final melt, g WO.sub.3 = 31                                                   X-ray diffraction                                                             Major WC, trace C                                                             No W.sub.2 C detected                                                 Spectrographic analysis of WC product, pct                                    Al   Cu     Fe     Mg   Mn   Mo    Si   Sn   Ti   V                           ______________________________________                                        0.005                                                                              0.007  0.10   0.005                                                                              0.05 <.001 0.07 0.009                                                                              0.01 0.006                       Other impurities below detection limits.                                      Carbon and oxygen analysis, pct                                               C = 6.19                                                                      O = 0.34                                                                      Specs for Commercial WC                                                       Max. pct                                                                             Free carbon                                                                            0.10                                                                 Fe       0.15                                                                 Mo       0.10                                                          Stoichiometric carbon content of WC = 6.13 pct.                               ______________________________________                                    

EXAMPLE 2

    ______________________________________                                        Feed material - California scheelite concentrate                              Analysis, pct                                                                 WO.sub.3 CaO Fe.sub.2 O.sub.3                                                 69.3 18.1 2.6                                                                 High Temperature phase separation                                                    Charge grams                                                           ______________________________________                                               Scheelite                                                                            432                                                                    NaCl   420                                                                    NaF    201                                                                    Al.sub.2 O.sub.3                                                                      72                                                                    Al.sub.2 SiO.sub.3                                                                   366                                                                           1,491                                                           ______________________________________                                    

The phase separation was made with the same procedure as that of Example1.

The molten phases were decanted into separate slag pots.

    ______________________________________                                        Analysis of the phases, pct                                                          Wt. g        WO.sub.3                                                                             CaO                                                ______________________________________                                        Halide   909            31.5   0.22                                           Silicate 515            2.0    14.5                                                    1,424                                                                ______________________________________                                    

Gas Sparging of Halide Phase

The halide phase containing 96 pct of the WO₃ from the scheelite wastreated by natural gas sparging. The natural gas contained, in pct:methane, 93.63; carbon dioxide, 0.70; nitrogen, 0.47; ethane, 3.58;propane, 1.02; other hydrocarbons, 0.60.

A 600-gram charge was melted at 1,100° C. in a 3" I.D.×7" high graphitecrucible. A 1/4" O.D.×1/8" I.D. Alundum tube was used for gas spargingthe melt. Natural gas was passed through the melt for 6 hours at a rateof 4 liters/min STP. The tube was removed and the remaining molten saltin the crucible was decanted into a slag pot. The WC was recovered fromthe cooled crucible by water leaching and scraping. The WC product wasleached in dilute HCl and NaOH solutions, and treated for free carbonremoval in heavy liquids.

    ______________________________________                                        Results                                                                       ______________________________________                                                WC recovered = 120 g                                                          W recovered as WC, pct = 75                                                   Final melt, g = 360                                                           Final melt, pct WO.sub.3 = 9.5                                                Final melt, g WO.sub.3 = 34.0                                                 x-ray Diffraction of WC product                                               Major WC                                                              Spectrographic analysis of WC product, pct                                    Al   Cu     Fe     Mg   Mn   Mo   Si   Sn   Ti   V                            ______________________________________                                        0.005                                                                              0.007  0.1    0.005                                                                              0.05 0.001                                                                              0.05 0.003                                                                              0.005                                                                              <.006                        Other impurities below detection limits.                                      Carbon and oxygen analysis, pct                                               C = 6.31                                                                      O = 0.32                                                                      ______________________________________                                    

EXAMPLE 3 Gas Sparging of Halide Phase Prepared from Wolframite in aMetal Alloy Crucible

A 1,600 gram charge of halide phase prepared according to the procedureof Example 1, and containing 25 wt pct WO₃, was melted in an Inconel 600crucible 3" I.D.×12" high. A 1/4" Inconel pipe was the sparging tube.The melt was heated to 1,070° C. and methane sparged for 4 hours at agas flow rate of 5 liters per minute STP. The WC was recovered bydecanting off the unreacted melt, cooling the crucible and waterleaching and scraping. The WC product was leached in dilute HCl and NaOHsolutions, water washed and dried.

    ______________________________________                                        Results                                                                       ______________________________________                                                WC recovered = 181.5 g                                                        W recovered as WC, pct = 54                                                   Final melt, g = 992                                                           Final melt, pct WO.sub.3 = 15.5                                               Final melt, g WO.sub.3 = 153.8                                                X-Ray Diffraction                                                             Major WC                                                              Spectrographic analysis of WC product, pct                                    Al   Ca     Cr     Cu   Fe   Mg   Mn   Mo   Na   Ni  Si                       ______________________________________                                        .005 <.01   0.1    .002 .04  .001 .001 .05  .04  .02 .02                      Other impurities below detection limits.                                      Carbon and oxygen analysis, pct                                               C = 6.19                                                                      O = 0.07                                                                      ______________________________________                                    

The Inconel crucible was used a total of 5 times with similar results. Amajor advantage of using the Inconel crucible was that free carbon wasnot detected in the WC product, thus eliminating a free carbon cleaningstep.

EXAMPLE 4

    ______________________________________                                        WC from Na.sub.2 WO.sub.4                                                            Charge grams                                                           ______________________________________                                               NaCl   440                                                                    Na.sub.2 WO.sub.4                                                                    160                                                                           600                                                             ______________________________________                                    

Procedure

The charge was melted in an alumina ceramic crucible 23/4" I.D.×6" high,at 1,100° C. Methane gas was passed through the melt for 6 hours at aflow rate of 4 liters/min STP.

    ______________________________________                                        Results                                                                       ______________________________________                                                WC recovered = 92 g                                                           W recovered as WC, pct = 86                                                   Final melt, g = 260                                                           Final melt, pct WO.sub.3 = 6.2                                                Final melt, g WO.sub.3 = 16                                           Spectrographic analysis of WC product, pct                                    Al B   Cu     Fe     Mg Mn  Na  Pb   Si  Sn    TiZr                           ______________________________________                                        0.05 0.004                                                                           0.01   0.03   0.02 0.02                                                                            0.4 <.01 0.1 <.002 0.01 0.1                       ______________________________________                                    

WC was also prepared in NaCl-Na₂ WO₄ melts containing 50 pct WO₃. The WCyield was about 50 pct.

EXAMPLE 5

    ______________________________________                                        WC from WO.sub.3                                                                     Charge grams                                                           ______________________________________                                               NaCl   500                                                                    WO.sub.3                                                                             100                                                                    Na.sub.2 CO.sub.3                                                                     46                                                                           646                                                             Procedure:                                                                    Temperature, 1,100° C.                                                 Graphite crucible, 3" I.D. × 7" high                                    1/4" alumina sparging tube                                                    Methane gas sparged for 6 hours at                                            a flow rate of 3 liters/min STP                                               Results                                                                       WC recovered = 68 g                                                           W recovered as WC, pct = 80                                                   Final melt, g = 398                                                           Final melt, pct WO.sub.3 = 5.0                                                Final melt, g WO.sub.3 = 19.9                                                 ______________________________________                                    

The analyses were similar to those obtained in Example 4.

EXAMPLE 6

    ______________________________________                                        WC from ammonium paratungstate (89 pct WO.sub.3)                                      Charge                                                                              grams                                                           ______________________________________                                                NaCl  500                                                                     APT   140                                                                     NaOH   43                                                                           683                                                             ______________________________________                                    

Procedure

The charge was melted in a 3" I.D.×7" high graphite crucible at 1,100°C. Methane gas was passed through a 1/4" alumina tube into the melt for6 hours at a flow rate of 4 liters/min STP. Ammonia gas was given offduring heating of the APT prior to melting.

    ______________________________________                                        Results:                                                                      ______________________________________                                                WC recovered = 74 g                                                           W recovered as WC, pct = 70                                                   Final melt, g = 307                                                           Final melt, pct WO.sub.3 = 7.5                                                Final melt, g WO.sub.3 = 23.0                                         ______________________________________                                    

The analyses were similar to those obtained in Example 4.

We claim:
 1. A process for preparation of tungsten monocarbidecomprising:(a) providing a molten composition comprising an alkali metalhalide and an oxygen compound of tungsten; (b) sparging said compositionwith a gas comprising a gaseous hydrocarbon which is selected from thegroup consisting of natural gas, methane, ethane, acetylene, propane,butane, mixtures thereof, and admixtures of these gases with H₂ or CO,at a temperature of about 900° to 1100° C. for a sufficient time for thetungsten compound to be substantially converted to tungsten carbide; and(c) decanting the molten halide from the tungsten carbide product. 2.The process of claim 1 wherein the alkali metal halide is sodiumchloride.
 3. The process of claim 1 in which the oxygen compound oftungsten is tungstic oxide, an alkali metal tungstate, or ammoniumtungstate.
 4. The process of claim 3 in which the tungsten compound issodium tungstate.
 5. The process of claim 1 in which the gaseoushydrocarbon is methane or natural gas.
 6. The process of claim 1 inwhich the tungsten carbide product is cooled and purified by waterleaching.
 7. The process of claim 1 in which the molten compositionconsists essentially of a mixture of sodium chloride and sodiumtungstate.
 8. The process of claim 7 in which the NaCl-Na₂ WO₄ mixtureis prepared by forming a melt comprising a tungsten ore concentrate,sodium chloride and sodium silicate, whereby the tungsten concentratesin the sodium chloride phase.
 9. The process of claim 8 in which thetungsten ore is scheelite or wolframite. .Iadd.
 10. The process of claim1, wherein a reductant is included in the molten composition. .Iaddend..Iadd.11. The process of claim 10, wherein the reductant is selectedfrom the group consisting of powdered charcoal, coke breeze and calciumcarbide. .Iaddend.